Providing at least a limited form of communication via powerlines, i.e., transmission lines and associated infrastructure deployed for the primary purpose of distributing electrical energy, has been around for approximately 40 years. For example, communication over powerlines was initially utilized by power transmission companies for monitoring and controlling power transmission infrastructure, such as to monitor power transmitted through particular nodes in the power grid and/or to control power grid switches when a fault or overload condition is detected. Additionally, power transmission companies have occasionally utilized such communications to transmit voice, such as between a field service technician and a distribution station operations center to facilitate the installation or maintenance of power transmission equipment.
Accordingly, those providing such communications were primarily interested in the long distance transmission lines, e.g., in the range of 50 kilometers, such as might be associated with the high voltage transmission lines connecting a power distribution station to a power generation plant or power distribution sub-stations. Typically, in providing communications via powerlines over the above described distances, power transmission companies utilized very low frequencies, such as on the order of a few kilohertz, in order to minimize attenuation of the communication signal. Accordingly, only a very small bandwidth was available for communications, although such a small bandwidth was sufficient for the relatively limited amount of communication required for monitoring, control, and voice signaling implemented in the past.
The use of such powerlines for transmission of communication signals was, and is, impeded by characteristics of the power line environment in addition to the aforementioned signal attenuation limiting bandwidth. For example, power transmission systems are typically replete with transformers disposed in the power line transmission path to provide voltage conversion. Specifically, transmission of a high current through a transmission line results in resistive losses greater than transmission of a high voltage through the same transmission line. Accordingly, transmission of a desired amount of power may be accomplished more efficiently by transmitting a relatively high voltage with a correspondingly relatively low current. However, to provide the desired power at the loads the voltage generally needs to be relatively low and/or the current relatively high and, therefore, power transmission systems include a number of transformers disposed throughout, such as transformers at power distribution sub-stations to transform high voltage (e.g., 350 kilovolts) transmitted from a power distribution station to a medium voltage (e.g., 30 kilovolts) and transformers at subscriber distribution nodes to transform medium voltage transmitted from a power distribution sub-station to a low voltage (e.g., 110 volts) for subscriber use. Each such transformer presents a potential choke or block to communication signal transmission, even the relatively low frequency communication signals described above. Specifically, these transformers are designed to very efficiently pass energy at the native frequency of the transmitted power, e.g., 60 Hz in the United States or 50 Hz in Europe, but effectively provides a filter with respect to signals substantially outside of this native band.
Although early communication via powerlines was primarily utilized in situations in which the presence of such transformers did not significantly impede the communications, e.g., long distance communications between a power distribution station and a power distribution sub-station utilizing only the high voltage transmission lines therebetween, further penetration of transmission of communication signals required a solution to the blocking of communication signals by the transformers. Accordingly, power transmission line couplers were later developed to provide a communication signal by-pass path around such transformers.
It should be appreciated that providing a power transmission line coupler to establish an effective transformer by-pass path is typically very complicated and expensive. Specifically, the potential (voltage) difference between the two power transmission lines being coupled is generally very great, e.g., on the order of 300 kilovolts in some situations where high voltage is transformed to medium voltage and on the order of 30 kilovolts in some situations where medium voltage is transformed to low voltage. Accordingly, a coupler for use in such a communication system is required to provide communication signal transmission while providing protection or isolation of the greatly different voltages. The solutions developed to meet such criteria have generally been unable to accommodate a high frequency communication signal transmission and, therefore, have continued to limit power line communication signals to low frequencies and low bandwidths. Moreover, due at least in part to the expense, complexity, and reliability of such transmission line couplers, these couplers have not been utilized to provide communications via powerlines down to the power subscriber level.
In an alternative solution to the blocking of communication signals by the transformers, later implementations have utilized wireless communication links. For example, a power transmission company may utilize the aforementioned high voltage transmission lines to carry communication signals relatively long distances and, before encountering a transformer in the communication path, deploy a wireless transmitter to complete the last portion of the communication path, e.g. that associated with medium voltage transmission lines. Generally, the communication path associated with the medium voltage transmission lines is shorter than that of the high voltage transmission lines. For example, it may be desired to control a switch, control a capacitor bank for power factor control, or monitor power transmission associated with a medium voltage portion of the power grid. Medium voltage portions of the power grid are typically associated with a relatively small area corresponding to an area served by a power transmission sub-station. Accordingly, a wireless transmitter may be coupled to a high voltage transmission line at a power transmission sub-station and utilized for providing communication links throughout much of the area served by that sub-station.
Additionally, wireless solutions have been implemented in order to provide communication at the subscriber level of the power transmission system. For example, power meters having a wireless transmitter have been deployed at subscriber sites to provide communication of meter readings. According to one embodiment, wireless receivers are deployed upon service vehicles, such as garbage trucks, which regularly canvas subscriber areas. As such vehicles pass within range of power meters having such wireless transmitters, the receivers may receive and record information, such as the current meter reading. The wireless receivers may then be linked to a power transmission company system, such as via a dial-up or wide area network connection, at a service vehicle depot, e.g., the city garage. Although providing communication of information from a subscriber site to the power transmission company, this solution suffers from several disadvantages. The system requires relatively expensive wireless communication systems to be disposed at every subscriber location to which communications are to be established as well as upon a typically large number of corresponding service vehicles. This system does not leverage any of the existing power transmission system infrastructure. Moreover, the communication of data from the subscriber site to the power transmission company is substantially delayed and, therefore, real-time monitoring of conditions, such as power outages, is not possible.
Another solution for providing penetration of communications to the subscriber level has been to implement a very low speed communication system. Specifically, power meters have been deployed which communicate their reading information via powerlines using a very low frequency transmission. Accordingly, such communication signals can traverse transformers optimized for very low frequency power transmission and, therefore, the use of the aforementioned transmission line couplers at subscriber distribution points may be foregone. However, such systems provide a very low bandwidth, such as communicating 48 bytes of data in a 24 hour period. This very low bandwidth may be acceptable for use in limited situations, such as transmitting the power meter reading once a month, but is not acceptable for providing a wide variety of communications.
It can be seen from the foregoing that power line communication systems have traditionally not been effective at penetrating the subscriber level. However, there are a number of advantages in providing such communication to the subscriber sites. In addition to the aforementioned ability to read the power meters at subscriber sites, such as for monthly billing purposes, it may also be advantageous to provide communication at the subscriber level in order to control power consumption peaks and/or to monitor operation of the power grid. For example, a power transmission company may wish to provide energy savings and/or avoid the need to obtain new sources of electric power by controlling subscriber loads. Specifically, as periods of peak energy consumption are detected, a power transmission company may wish to reduce non-essential use of electricity, such as by turning off a hot water tank, an air conditioning compressor, a swimming pool pump, or the like at a subscriber home or office. Likewise, a power transmission company may wish to control the subscriber load, such as through adjusting capacitors, to increase efficiency of the consumption of power by the subscriber.
Additionally or alternatively, a power transmission company may wish to monitor operation of the power grid as seen by the subscriber in real-time. Currently, a power transmission company is made aware of a power grid fault or other interruption in service through subscriber complaint. However, through the use of real-time communications at the subscriber level, the power transmission company may monitor interruptions in service and even the quality of service as experienced by the subscribers without subscriber interaction.